It is known to utilize active control of the combustion process in order to achieve stable, efficient, regulation-compliant operation of a gas turbine engine. Active control systems typically include a sensor for detecting a parameter responsive to the combustion process and a control element for controlling the combustion process in response to the measured parameter. One or more such parameters may be measured; for example, the parameter may be the pressure in the combustion chamber, a predetermined wavelength of electromagnetic energy generated by the combustion process, or the exhaust concentration of a particular combustion byproduct such as carbon monoxide.
One such gas turbine with active combustion control is described in U.S. Pat. No. 5,428,951, incorporated by reference herein, wherein a pressure transducer is used to detect combustion-induced pressure oscillations and a flame kernel pulse actuator is controlled in response to the measured pressure oscillations. A flame kernel pulse actuator generates periodic flame kernels that function to suppress the combustion-induced pressure oscillations.
U.S. Pat. No. 4,557,106, incorporated by reference herein, describes an active combustion control system wherein a microphone is used to sense reheat buzz in a gas turbine exhaust stream. The measured sound parameter is used as an input to a vibrating diaphragm that alters the boundary conditions of the combustion and thereby prevents resonance from building. This patent further describes embodiments of the invention wherein the sensor may be a pressure transducer or a photo detector.
In each of the known active combustion control systems, the sensor utilized to measure the combustion control parameter must be positioned proximate the combustion chamber. The exact location and orientation of the sensor may vary from application to application, but there is often a requirement for direct contact between the tip of the sensor and the hot combustion gas. The above-cited patents depict their respective mounting arrangements in schematic fashion and do not teach or suggest the difficulties that may be encountered in securing the sensor in the correct position.
U.S. Pat. No. 5,706,643, incorporated by reference herein, illustrates another active gas turbine control system wherein a pressure probe is illustrated as being mounted to a combustor wall, with a pressure signal being routed to the outside of the engine case by an electrical wire connection. Replacement of such a pressure probe would require the disassembly of the engine case, and the wire connection may be vulnerable to mechanical damage.
U.S. Pat. No. 5,544,478, incorporated by reference herein, illustrates a system that utilizes an optical sensor installed in a combustor wall and extending into a flame shield wall. It is unclear from the patent whether the sensor is free to move in relation to the flame shield wall. If the sensor is designed to move, the alignment of the openings in the respective walls must be held to a close tolerance to ensure free movement of the sensor. If the sensor is not free to move, it may be subject to a high level of stress resulting from the differential thermal growth of the two wall members.
FIG. 1 illustrates a prior art gas turbine engine 10 sold by the assignee of the present invention, Siemens Westinghouse Power Corporation. The engine 10 includes a combustion chamber 12 defined by a combustor wall 14. A burner assembly 16 provides a mixture of fuel and air for combustion in the combustion chamber 12. These components are housed within a casing 18 that forms the pressure boundary of the machine. A pressure transducer 20 is mounted outside the casing 18 for measuring pressure fluctuations within the combustion chamber 12 for use with an active combustion control system. The transducer 20 is installed into a mounting block 22 having a fluid connection to the combustion chamber 12. The fluid connection is established through a hole 24 formed through the “top hat” of the casing 18 to which is connected a communication tube 26. The communication tube extends along the longitudinal length of the burner assembly 16 and is connected, such as by welding, to a port 28 formed on the combustor wall 14. The port 28 is installed in the same axial plane as an end of a pilot cone 30 of the burner assembly 16. A ½ inch weldable Swagelok® fitting 32 is welded to the casing 18 and the mounting block 22 is connected to the fitting 32. A damping tube 34 is connected to the opposed side of the mounting block 22 via a ½ inch Swagelok® union 36 in order to eliminate the acoustic resonance inherent in the length of the communication tube 28. This arrangement has the advantage of keeping the transducer 20 outside of the casing 18, which simplifies replacement of the transducer and avoids exposure of the transducer 20 to the environment inside the engine 10. Furthermore, the communication tube 26 is generally protected from physical damage by the casing 18 and it is sufficiently long in its axial dimension so that it can bend to relax any stresses caused by thermal expansion and contraction of the burner assembly 16. This system permits the pressure internal to the engine 10 to extend beyond the casing 18, and therefore, the pressure boundary created by the mounting block 20, fitting 32, union 36 and damping tube 34 must be properly protected against failure.